PROJECT SUMMARY ABSTRACT Syphilis, a multi-stage sexually transmitted disease caused by the spirochete Treponema pallidum (TPA), continues to be a major worldwide public health problem. Despite the wide availability of antibody (Ab)- based screening tests and effective antimicrobial therapy, syphilis control strategies have met many challenges, particularly in resource-poor countries where proper diagnosis and partner services to reduce transmission are not always feasible. The inability of traditional public health prevention strategies to curtail the spread of venereal syphilis underscores the critical need to develop a safe and effective vaccine that has global coverage. The current proposal (U19- CRC Project 2) embodies a novel translational approach that will greatly contribute towards syphilis vaccine development that builds upon our extensive understanding of the basic membrane and molecular biology of the syphilis spirochete (U19 CRC Project 1) and reverse vaccinology methods that can interrogate the antibody (Ab) responses the bacterium elicits in its human host to distinct and unique TPA outer membrane (OMP) targets. It is our contention that the characterization of TPA OMP antigenic targets and their variants, on a global scale, will lead us to syphilis vaccinogens whose ability to induce opsonic antibodies and protection can be tested (Aims 1, 2 and 3). Two major developments stemming from our microbiologic, immunologic and translational studies position us to test our unifying hypothesis. First, we have made significant progress characterizing TPA's repertoire of rare OMPs (U19 CRC Project 1) and second, we have made refined existing and developed new methods to assess the presence of opsonic anti-OMP antibodies in patient sera and how antigenic diversity in these targets affects their ability to recognize circulating TPA (Project 2 and 3). Collectively, our syphilis clinical network in the United States, Colombia, Malawi and China, steered by an expert group of physician scientists, epidemiologists, immunologists and microbiologists, genomics and analytics experts, lay the necessary groundwork to enroll and study early syphilis patients at a global scale (Aim 1), collect and extract TPA DNA to understand the complete assortment of OMPs that are expressed globally by circulating TPA (Aim 2), and understand if syphilitic Abs are indeed able to recognize candidate vaccine antigens and their antigenic variants (Aim 3). In the end, our approach will lend itself to formulate and test candidate TPA OMP antigens and model the effects of such a vaccine that can be used globally.